The halogen atoms of α-halo sulfones, in contrast to halogen atoms α to other electron-withdrawing functionalities, show marked resistance to substitution by external nucleophiles. Seemingly, polar, steric, and field effects combine to repel nucleophilic species. However, the same α-sulfonyl halogen atoms are capable of facile intramolecular 1,3 elimination, leading to replacement of the sulfonyl group by a carboncarbon double bond with loss of halide and sulfite ions. This extrusion process, frequently referred to as the α-halo sulfone or Ramberg-Bäcklund rearrangement after its discoverers, has found broad utility in olefin synthesis. The reaction is generally applicable and easy to use.
Its earliest application dealt with the preparation of alkenes, the cis isomers of which predominated. Whereas such molecules may be more readily available by other methods, none of the alternative procedures offers the added option of specifically replacing the olefinic hydrogen atoms with deuterium by merely conducting the rearrangement in deuterated solvents. Moreover, because the α-halo sulfones undergo this transformation in alkaline solution, further rearrangement of the initially formed alkenes is precluded. Therefore the SO2 group in the starting sulfone is invariably replaced cleanly and unequivocally by the π bond.
The rearrangement is generally applicable to molecules containing the minimal structural requirements of a sulfonyl group, an α-halogen atom, and at least one α′-hydrogen atom, even in systems leading to small-ring cycloalkenes. Di- and tri-halo sulfones behave analogously. Conformational constraints, adverse hybridization characteristics, and excessive strain are known to deter the reaction. Inasmuch as these features arise only in special circumstances, they do not necessarily detract from its usefulness.
This chapter summarizes the more important advances in the understanding of α-halo sulfone rearrangements, particular consideration being given to the nature of the intermediates, scope of the available synthetic alternatives, optimization of experimental conditions, and effect of structural features on reactivity. Closely related transformations are also discussed.